Uh... Mega-Lucario, uber?
Nah.
It's OU and pretty strong, but not an Uber indeed.
Talonflame and Aegislash are quite diffused and counter it 9/10 times, so...
There also are many other pokémon who could take it without problems. Seriously. Just switch in at the right time, take one hit and 1HKO it. It's not difficult.
It's just that people, especially on Showdown, always use the same pokémon with the same sets which happen to be unreliable against M-L. >_> Seriously, I get always insulted when I get around 1700s in Showdown because I use uncommon sets that people can't predict. .__.
Honestly speaking, I didn't have a problem with Mega-Gengar either.
Mega Kangaskhan and Blaziken were definitely uber though, yes.